PROJECT ABSTRACT Sensorineural hearing loss, the most common form of hearing loss, is caused by degeneration, dysfunction, or malformation of sensory hair cells or spiral ganglion neurons in the inner ear. While specification of spiral ganglion neurons during development is critical for transmitting auditory information to the brain, we lack a complete understanding of the mechanisms by which they are induced and establish synaptic connections. Pluripotent stem cells (PSCs) serve as a promising tool to model inner ear development and produce replacement sensory cells for cellular therapy. However, the controlled derivation of inner ear cells from human PSCs remains a significant challenge. We recently described a method for deriving inner ear organoids from mouse PSCs in a three-dimensional culture system. Mimicking normal development, the organoids started as otic vesicles and progressively formed neurons and sensory epithelia containing hair cells. In this study, we will build on preliminary studies of a novel human organoid culture system to investigate the mechanisms of neural induction from otic progenitors. Here, we hypothesize that dynamic Wnt, Notch, and fibroblast growth factor (FGF) signaling activity is critical for otic neural progenitor specification. In Aim 1, we will define the origin and type of inner ear neurons produced by human otic progenitors, in response to Wnt activation, by tracking the developmental stages. In Aim 2, we will visualize and quantify neural progenitors in stem cell-derived otic epithelia by using CRISPR/Cas9 gene editing to engineer a novel reporter cell line. Using this reporter cell line, we will systematically examine the roles of Notch and FGF signaling pathways in neural progenitor specification. Together, we anticipate that the completion of these aims will establish a novel in vitro model of human inner ear neurogenesis. Moreover, our long-term goal is to apply the optimized technical approach established in this study to future studies on interrogating the mechanisms and transcriptional regulators governing auditory circuit formation.